The present invention relates generally to AC coupling circuits and more specifically to a constant input impedance AC coupling circuit for a current measurement probe.
Current probes measure the flux field generated by the movement of electrons through a conductor. The flux field surrounding the conductor is converted to a linear voltage output that can be displayed and analyzed on a measurement test instrument, such as an oscilloscope. One type of current probe is an AC only probe. AC only probes are configured with either a solid core or a split core and are passive devices that do not require external power. AC/DC current probes generally have a split core configuration and include a Hall Effect device for producing a voltage output in response to a DC generated flux field.
FIG. 1 illustrates a simplified AC/DC current probe system 10 based on the A6312 current probe, the AM503B Programmable Current Probe Amplifier and TM500 Power Module manufactured and sold by Tektronix Inc., Beaverton Oreg. The TM500 Power Module provides electrical power to the AM503B Programmable Current Probe Amplifier. As shown in FIG. 1, the current probe 12 has a split core 14 of magnetic material defining an aperture 16 through which a conductor 18 carrying a current to be measured extends. A multi-turn winding 20 is wrapped around one leg of the core 14. A thin film semiconductor Hall Effect device 22 is disposed within the magnetic core 14. A bias source 24 housed in the current probe amplifier 26 provides power for the Hall Effect device 22 via a multi-conductor cable 28. The Hall Effect device 22 provides a differential input signal to a Hall pre-amplifier 30 in the current probe amplifier 26 via the multi-conductor cable 28. The output of the Hall pre-amplifier 30 is applied to a power amplifier 32 that is provided with a feedback resistor 34. The output of the power amplifier 32 is connected via the multi-conductor cable 28 to one end of the multi-turn winding 20 and the opposite end of the winding 20 is connected via the multi-conductor cable 28 to a low input impedance scaling circuit 36. The input to the scaling circuit 36 is terminated by resistor 38 having a value of 25 ohms. An AC/DC switching circuit 40 is positioned between the non-inverting input terminal of a differential scaling amplifier 42 and the terminating-load resistor 38. The switching circuit 40 selectively couples an AC coupling capacitor 44 into the input line of the scaling amplifier 42. The output of the scaling amplifier 42 is into a 50 ohm environment which is coupled via a coaxial cable 46 to the 50 ohm input resistor 51 of the measurement test instrument 48, such as an oscilloscope. The front panel 50 of the current probe amplifier 26 includes buttons, knob, LEDs, numerical readout and input and output connectors for controlling the operation of the amplifier and coupling the current probe 12 and measurement test instrument 48 to the amplifier 26. Depressing the appropriate buttons on the current probe amplifier 26 apply signals to a controller 52 that selectively couple the DC or AC signal path the input of the scaling amplifier 42 and generates a digital output to a digital-to-analog converter 54 to vary the gain of the scaling amplifier 42.
The oscilloscope is set to DC coupling and 10 millivolts per division scale and coupled to the current probe amplifier 26 via the coaxial cable 46. The current probe 12 is coupled to the current probe amplifier 26 via the multi-conductor cable 28. An operator selects AC or DC coupling and the gain for the scaling amplifier 42 using the front panel 50 controls. The gain of the scaling amplifier 42 varies in a 1-2-5 sequence from 1 to 500 and is displayed on the numerical readout as current per division. The current carrying conductor is inserted through the aperture 16 of the split magnetic core 14. The high frequency component of the current in the primary conductor 18 results in a current being induced in the secondary winding 20 in a direction such as to generate a magnetic field in the core 14 that is opposed to the field created by the current in the primary conductor 18. The low frequency or DC component of the current in the primary conductor 18 is less effective at inducing current in the secondary winding 20, but generates a potential difference across the Hall Effect device 22, and the amplifier 32 provides a corresponding current in the winding 20. The direction of the current supplied by the amplifier 32 is such that the magnetic field created in the core by the current flowing through the winding 20 is opposite to the direction of the magnetic field created by the current in the primary conductor 18. Over a wide range of frequencies, the voltage developed across the load resistor 38 is representative of the current in the primary winding 20.
The voltage developed across the load resistor is coupled to the high impedance input of the scaling amplifier 42. The scaling amplifier 42 amplifiers the input voltage by the amount of gain set by the operator. The output signal of the amplifier 42 is coupled to the low input impedance input of the oscilloscope. The oscilloscope processes the signal from the current probe amplifier 26 and produces a trace on the oscilloscope display representing the current signal in the primary conductor 18. To determine the amplitude of the current signal, an operation estimates the amount of vertical deflection of the signal in vertical divisions of the oscilloscope, for example 1.5 divisions. The vertical division number is divided by scale setting of the oscilloscope (i.e. 10 mv/div) and multiplied by the current per division setting of the current probe amplifier 26 (e.g. 20 ma/div) to produce the amount of current flowing through the primary conductor 18.
What is needed is a current probe amplifier that allows the scaling circuitry of the measurement instrument to provide the current per division scaling for current measurements. This requires a current probe amplifier that couples the current output of the current probe directly into the low input impedance input of the measurement instrument while maintaining a constant input impedance.
Accordingly, the present invention is a constant input impedance AC coupling circuit for a current probe measurement system. The current probe measurement system has a current measurement probe generating a current output signal via transformer action with a current carrying signal conductor and a Hall Effect device disposed in the core of the transformer providing a DC or low component of the current carrying signal conductor. The constant input impedance AC coupling circuit couples the current output signal from the current measurement probe to a resistive terminating element of a low input impedance measurement instrument. The constant input impedance AC coupling circuit has a capacitor coupling the current output signal of the current measurement probe to the low input impedance measurement instrument. The capacitor forms part of a resistive-capacitive network that includes the resistive terminating element. The resistive-capacitive network has a low frequency cutoff, typically less than 10 hertz, and a RC time constant. A resistive-inductive network is coupled to the resistive-capacitive network and receives the current output signal from the current measurement probe for terminating DC and low frequency signal components of the current output signal below the low frequency cutoff of the resistive-capacitive network in the same low input impedance of the measurement instrument. The resistive-inductive network provides a current path for shunting the DC and low frequency signal components to prevent transformer saturation of the current measurement probe. The resistive-inductive network has a synthesized inductor with a high inductive value, large current carrying capacity and an L/R time constant equal to the RC time constant of the resistive capacitive network. In the preferred embodiment of the invention, the synthesized inductor is implemented as a generalized impedance converter in the form of a gyrator. In a further embodiment of the present invention, an isolation inductor maybe coupled in series with the synthesized inductor to isolate the resistive-inductive network from the AC coupling circuit at higher frequencies.
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with appended claims and attached drawings.